<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

    <div>
        圆的半径：<input id="r" type="text">
    </div>
    <div>
        圆的周长：<input id="c" type="text">
    </div>
    <div>
        圆的面积：<input id="area" type="text">
    </div>
    <script>
        var r = prompt("请输入圆的半径");
        if (!isNaN(r)) {
            var c = 2 * Math.PI * r;
            var area = Math.PI * r * r;
            document.getElementById('r').value = r;
            document.getElementById('c').value = c.toFixed(2);
            document.getElementById('area').value = area.toFixed(2);
        } else {
            alert("请输入正确的数字")
        }
    </script>
</body>

</html>